1. Field of the Invention
This invention relates to an improved process for the production of coatings on heat-resistant substrates, in particular metal wires, using stoving lacquers based on polyisocyanates having at least partially blocked isocyanate groups and compounds having isocyanate reactive groups, which lacquers contain N,N',N"-tris-(2-hydroxyethyl)-isocyanurate as additive, and the use of the said compound as additive in such lacquers.
2. Description of the Prior Art
It is already known to coat heat-resistant substrates using coating compounds based on combinations of organic polyisocyanates having at least partially blocked isocyanate groups and compounds having isocyanate reactive groups.
These processes known in the art are suitable in particular for wire lacquering (see e.g., German Auslegeschrift No. 1,644,794, German Offenlegungsschriften Nos. 2,404,739 and 2,404,740, German Auslegeschrift No. 2,626,175 or German Offenlegungsschrift No. 2,840,352). In the coating compounds used in these known art processes, the polyisocyanate component preferably consists exclusively of completely blocked polyisocyanates. The coating compounds are therefore stable in storage below 80.degree. C. and hardening is carried out only after the coating process has been completed, by heating the coating to temperatures above 80.degree. C. so that the blocking agent is split off.
This cross-linking by heat is accompanied by several processes. The solvent usually present evaporates and at the same time the binder components undergo a sharp drop in viscosity due to the rise in temperature before the cross-linking reaction takes place with release and evaporation of the blocking agent.
In order to obtain the best surface characteristics for the hardened film, it is very important that the events described above should take place in the optimum sequence. Thus, for example, extensive hardening before the solvent evaporates results in the formation of craters. On the other hand, cross-linking should not take place too late because otherwise the binder, which is highly fluid at high temperatures, alters its distribution over the substrate, for example due to gravity or surface tension, before it has become fixed on the substrate by hardening.
In view of the complexities regarding the events of film formation, it is extremely difficult to find measures which enable processing to be carried out more rapidly.
The choice of low boiling solvents and hence more rapid spread of the increase of temperature throughout the binder in most cases results in lacquer films with unsatisfactory surface characteristics. The same difficulty is also encountered if the specific catalysis of urethane formation is too vehement.
It has now surprisingly been found that the addition of N,N',N"-tris-(2-hydroxyethyl)-isocyanurate (THEIC) to coating compounds, i.e. the addition of THEIC as a binder component, results in a very substantial improvement in the processing characteristics of the coating substance. The addition of this compound in particular enables the speed of transport of metal wires through the usual apparatus used for wire lacquering to be substantially increased without the quality of the lacquering obtained being in any way deleteriously affected.